This invention relates to an apparatus and method for coupling signals onto a transmission line. It has particular application to coupling radio frequency (RF) signals onto an electricity distribution network which is used to transport telecommunications signals.
It is known to transport telecommunications signals over an electricity distribution or power transmission network. Patent Application WO 94/09572 A1 (NORWEB) describes such a network. Delivering a telecommunications service in this manner is attractive as it overcomes one of the greatest costs in providing a new telecommunications network i.e. installing cabling to each subscriber. Existing electricity distribution cabling is used to carry the telecommunications signals.
FIG. 1 shows an electricity distribution network which is adapted to carry telecommunications signals. Mains electricity enters the network from an 11 kV transmission line 105 and is transformed by substation 100 into a 415V supply which is delivered over cable 120 to subscribers S. A base station BS couples telecommunications signals VB, such as voice and data signals, at injection point 110 onto distribution cable 120. The telecommunications signals propagate over the cable on radio frequency carriers to transceiver units TRX at subscriber premises S.
One of the problems with transporting RF signals over the electricity distribution network is that of unwanted radiation of RF energy. The distribution network was not designed to carry RF signals.
Electricity distribution cables, such as cable 120, have a concentric structure. The inner section of the cable comprises groups of conductors which carry one or more of the three supply phases. This inner section is surrounded by an outer section which is coupled to earth. These cables have similar screening properties to coaxial cables, and conveniently this screening, coupled with the underground burial of the cables, is effective at the radio frequencies (RF) that are used for transporting telecommunications signals.
The internal wiring at subscribers"" premises S is unscreened, and could potentially cause radiation problems. However, by filtering off the RF signals at the point where the electricity feeder cable becomes unscreened radiation of RF signals is minimised.
The other significant point where radiation can occur is at the substations 100 where electricity is transformed from 11 kV to 415V. Substations have busbars which are typically mounted as a grid array on the substation wall. The busbars are shielded from view but frequently are electrically unscreened. This is because screening is considered unnecessary at the 50 Hz mains frequency. At RF frequencies the busbar array functions as an antenna, radiating the RF signals which it receives via the distribution cables into the surrounding area. This is a undesirable as it causes interference with equipment operating at these frequencies. This radiation may also violate regulations on Electromagnetic Compatibility (EMC).
One of the solutions to minimise radiation from the busbars is to screen the busbar array, or to screen the entire substation building. Some modern substations are equipped with metal casing around the busbars. However, the majority of substations are unscreened brick structures. It is undesirable to renovate all of these structures to improve their screening as it increases the cost of providing a telecommunications service over the network.
An alternative solution to the radiation problem is to restrict the power at which the RF signals are transmitted over the network, such that radiation occurring at substations falls below acceptable limits. This causes problems with subscribers"" equipment, particularly to those subscribers furthest from the point at which RF signals are injected onto the network. Subscriber equipment needs an acceptable signal to interference ratio in order to detect the wanted RF signals. With considerable interference on the network, this demands a reasonably high transmit power.
The problem of radiation at the substation is compounded by the fact that RF signals are usually injected onto the distribution network adjacent to the substation. The reason for injecting at this point is because one base station can easily be coupled to each of a group of 415V cables (120, 130, 140 in FIG. 1) which all converge at the substation.
A paper entitled xe2x80x9cAdaptive Interference Cancellation for Power Line Carrier Communication Systemsxe2x80x9d at pp. 49-61 of IEEE Transactions on Power Delivery, Vol 6, No 1, January 1991, addresses the problem of frequency reuse in a power line carder system. A portion of a transmitted signal or a first line section which leaks through a line trap onto a second line section is cancelled by applying a cancelling signal to the second line section.
DE 2 523 090 describes a directional signal generator which controllably propagates in ore direction along a line. This uses an attenuator network in series with the line.
The present invention seeks to minimise the above problem.
According to a first aspect of the present invention there is provided an apparatus for coupling signals to a line, the apparatus comprising:
an input for receiving a wanted signal;
a first means for coupling the wanted signal onto the line at a first position;
a cancelling means, coupled to the input, for deriving a cancelling signal from the wanted signal, the cancelling means being operable to phase-shift the wanted signal,
a second means for coupling the cancelling signal onto the line at a second position, spaced from the first position;
and wherein the apparatus is arranged so that the combination of the phase-shift and propagation delays experienced by the signals causes the wanted signal and the cancelling signal to destructively combine in a single direction of propagation along the line while enabling the wanted signal to propagate in the other direction along the line.
Preferably the cancelling means comprises a weight which is operable to scale the wanted signal in amplitude.
Preferably the spacing of the first and second couplers is substantially one quarter of a wavelength of the wanted signal. This maximises signal power in the wanted direction of propagation.
Preferably the apparatus also has a monitor for sensing the combination of the wanted and cancelling signals at a position on the line and feeding the sensed signal to a calculating means which controls the cancelling means. This allows a more effective cancellation.
The calculating means can perform an iterative technique in which perturbations are applied to the value of the weights and the sensed signal is monitored to determine the effect of the perturbations.
The calculating means may alternatively perform an iterative technique in which the sensed signal is correlated with a portion of the wanted signal to determine updated weight values.
Preferably the apparatus is used to couple telecommunications signals to a power line such as a distribution line of an electricity distribution network for serving a plurality of subscribers.
In the situation where the telecommunications signals are coupled onto the electricity distribution network at a position between an unshielded part of the network and the subscribers, and the wanted and cancelling signals destructively combine in the direction of the unshielded part, this prevents unwanted radiation of signals from the unshielded part of the network.
A further application of controlling the direction of propagation of signals along the line is in allowing a particular frequency band which is in use on one line to be reused on the other lines. This has a particular use where an electricity distribution network has several distribution lines served by a common substation.
A further aspect of the invention provides a method of coupling signals to a line, the method comprising:
receiving a wanted signal at an input;
coupling the wanted signal onto the line at a first position;
deriving, at a cancelling means, which is coupled to the input, a cancelling signal from the wanted signal, the cancelling means being operable to phase-shift the wanted signal;
coupling the cancelling signal onto the line at a second position. spaced from the first position;
and wherein the combination of the phase-shift and propagation delays experienced by the signals is arranged such that the wanted signal and the cancelling signal destructively combine in a single direction of propagation along the line while enabling propagation of the wanted signal along the line in the other direction.
A further aspect of the invention provides a method of coupling communications signals on to an electricity distribution network comprising a substation serving a plurality of distribution lines, the method comprising:
coupling communications signals occupying a frequency band to one of the lines by coupling a wanted communications signal onto the line at a first position and coupling a cancelling signal onto a line at a second position, spaced from the first position such that the wanted communications signal and cancelling signal destructively combine in a direction of propagation towards the substation; and,
reusing the frequency band for coupling different communications signals on to another one of the plurality of lines.